The invention relates to a radiation image storage panel favorably employable in a radiation image recording and reproducing method utilizing stimulated emission of a stimulable phosphor.
When the stimulable phosphor is exposed to radiation such as X-rays, it absorbs and stores a portion of the radiation energy. The stimulable phosphor then emits stimulated emission according to the level of the stored energy when the phosphor is exposed to electromagnetic wave such as visible light or infrared rays (i.e., stimulating light).
A radiation image recording and reproducing method utilizing the stimulable phosphor is widely employed in practice. The method employs a radiation image storage panel comprising the stimulable phosphor, and comprises the steps of causing the stimulable phosphor of the storage panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with a stimulating light to emit stimulated light; and photo-electrically detecting the emitted light to obtain electric signals giving a visible radiation image. The storage panel thus processed is then subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as xe2x80x9cstimulable phosphor sheetxe2x80x9d) has a basic structure comprising a support and a stimulable phosphor layer provided thereon. If the phosphor layer is self-supporting, the support may be omitted. On the free surface (surface not facing the support) of the phosphor layer, a protective film is generally placed to keep the phosphor layer from chemical deterioration or physical damage.
The stimulable phosphor layer generally comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor without binder. The phosphor layer containing no binder can be formed by vapor deposition process or firing process.
It is desired that the radiation image storage panel has a sensitivity as high as possible and further give a reproduced radiation image of high quality (particularly, in regard of sharpness and graininess).
As described above, the radiation image storage panel having a stimulable phosphor film (layer) can be prepared by a vapor deposition process (or vapor-accumulating process) such as a vacuum vapor deposition (evaporation) process or a sputtering process. In the evaporation process, for example, an electron beam or a resistance heater is applied onto a stimulable phosphor or its starting materials (i.e., evaporation source) to heat and vaporize the source, to deposit the vapor on a support sheet (i.e., substrate) to form a phosphor film on the substrate.
Thus formed phosphor film consists essentially of prismatic crystals of the stimulable phosphor without binder. In the phosphor film (layer), there are cracks among the prismatic crystals of the stimulable phosphor. For this reason, the stimulating rays are efficiently applied to the phosphor and the stimulated emission are also efficiently taken out. Hence, a reproduced radiation image of high sharpness can be obtained with high sensitivity.
As a stimulable phosphor suitable for the vapor deposition process, an alkali halide stimulable phosphor has been known. For example, JP-2,060,688 and WO 01/03156 disclose some alkali halide stimulable phosphors. However, there is given no description of oxygen in the phosphor layer of alkali halide stimulable phosphor.
An object of the present invention is to provide a radiation image storage panel giving a reproduced radiation image of high quality with high sensitivity.
According to the study of the present inventors on the preparation of a phosphor layer by vapor deposition, it has been noted that the sensitivity can be remarkably improved by incorporating a specific amount of oxygen evenly in the phosphor layer. This means that, in order to increase the amount of stimulated emission from the phosphor layer, it is important to make a certain amount of oxygen is present in prismatic phosphor crystals and further to submit the deposited phosphor layer to heat treatment by which oxygen atoms are separated from the activator (such as Eu) and diffused evenly in the phosphor layer. The oxygen atoms in the phosphor crystals compensate electric charges of the activators such as Eu, and hence the separation of oxygen from the activator improves efficiencies of activation and charge transfer of the activator. As a result, the amount of stimulated emission emitted from the phosphor layer is remarkably increased.
The present invention resides in a radiation image storage panel having a deposited phosphor layer, wherein the phosphor layer comprises an alkali metal halide stimulable phosphor having the formula (I):
xe2x80x83MIXxc2x7aMIIXxe2x80x22.bMIIIXxe2x80x33:yA, zOxe2x80x83xe2x80x83(I)
in which MI is at least one alkali metal element selected from the group consisting of Li, Na, K, Rb and Cs; MII is at least one alkaline earth metal element or divalent metal element selected from the group consisting of Be, Mg, Ca, Sr, Ba, Ni, Cu, Zn and Cd; MIII is at least one rare earth element or trivalent metal element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga and In; each of X, Xxe2x80x2 and Xxe2x80x3 independently is at least one halogen selected from the group consisting of F, Cl, Br and I; A is at least one rare earth element or metal element selected from the group consisting of Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, DY, Ho, Er, Tm, Yb, Lu, Na, Mg, Cu, Ag, Tl and Bi; and a, b, y and z are numbers satisfying the conditions of 0xe2x89xa6a less than 0.5, 0xe2x89xa6b less than 0.5, 0 less than y less than 1.0 and 0 less than zxe2x89xa60.2, respectively, and
oxygen contents represented by z1 and z2 at optionally determined two areas in the phosphor layer satisfy the condition of 0.2xe2x89xa6z1/z2xe2x89xa65.
The invention further resides in a process for preparing a radiation image storage panel having a deposited alkali metal halide stimulable phosphor layer on a support sheet which comprises the steps of:
depositing on the support sheet vapors of components of the alkali metal halide stimulable phosphor in an atmosphere having an oxygen partial pressure in the range of 1xc3x9710xe2x88x926 to 1xc3x9710xe2x88x922 Pa to form on the support sheet a deposited phosphor layer comprising an alkali metal halide stimulable phosphor having the aforementioned formula (I), and
annealing the deposited phosphor layer by heating the phosphor layer.